The world's most powerful production four-cylinder engine in a compact hot hatch. Perfect for enthusiasts who want supercar performance with daily usability. The M139 engine is surprisingly reliable when maintained properly, but the 8-speed dual-clutch gearbox can develop issues with harsh engagement from cold. Main concerns: gearbox crunching noises, dashboard rattles, coolant leaks from water pump location under exhaust manifold, ADAS sensor malfunctions in bad weather, and interior quality inconsistencies. Paint issues reported on Jupiter Red and Mars Red colors. The 4MATIC+ drivetrain with drift mode is complex but generally solid. Mandatory service every 10,000 km or 12 months. Avoid remapped cars as this overstresses the already-strained transmission.
Metallic crunching noise when engaging reverse from cold, jerky shifts · more· less
The 8-speed dual-clutch transmission (8DCT) has known issues with harsh engagement and crunching noises, particularly when shifting to reverse from cold. Software updates can sometimes cure it, but many cases require mechanical repair. The gearbox uses forks and gears like a manual rather than clutch packs. When testing, start the engine cold and cycle through N-R-N-D-N several times - any metallic crunching (not just a clunk) indicates major repair needed. If under warranty, Mercedes should replace the gearbox. Out of warranty, expect €2,500-4,000 for mechatronic unit repair or €6,000-8,000 for complete replacement. Labor is intensive due to transaxle removal. Quality variability exists - some units are flawless while others fail early.
Water pump/thermostat leak€1,500 - €3,500
Coolant leak from pump located beneath exhaust and turbo, labor-intensive repair · more· less
The water pump on AMG 45 models is positioned beneath the exhaust manifold and turbocharger, making it extremely difficult to access. Symptoms include coolant warning lights, visible coolant puddles, sweet smell from engine bay, or poor cabin heating. The repair requires disassembling the turbocharger, exhaust manifold, and disconnecting oil/coolant lines. Parts cost €400-600, but labor can run €1,000-2,000 at specialists and €2,000-3,000+ at dealers due to 8-12 hours of work. Often the thermostat housing is replaced at the same time. High-performance engines run hotter, accelerating pump seal degradation. Typically occurs between 70,000-100,000 km. Essential to use proper Mercedes coolant specification.
Dashboard and interior rattles€200 - €1,500
Constant rattles from MBUX screen, air vents, door cards - widespread quality issue · more· less
Interior rattles are extremely common on the W177 platform, with owners reporting buzzes and creaks from the dashboard center screen area, aluminum air vent rings, instrument cluster surround, door cards, and seats. The MBUX screen mounting system has screws clamping plastic to floating metal clips, allowing movement. Many owners report 3+ dealer visits attempting fixes with limited success. Some dealers apply insulating material behind the screen unit per Mercedes guidelines. DIY fixes include lubricating shoulder mounts and clips with silicone spray or petroleum jelly. Cold weather and rough roads exacerbate the issue. This is considered a build quality problem on Mercedes' cheapest platform (MFA2). Fixing properly can cost €500-1,500 at dealers for soundproofing and parts replacement, with no guarantee of permanent resolution.
Clearcoat bubbling and peeling on plastic parts, door handles, roof, spoiler · more· less
Jupiter Red and Mars Red colors have documented paint defect issues affecting A-Class, CLA, and GLA models including the A45. Paint lacquer bubbles form and spread across doors, A-pillars, roof, bonnet, and plastic parts like door handles and rear spoilers. Mercedes acknowledges Mars Red as defective (class action settlement in 2024 provides extended warranty coverage), but Jupiter Red is not officially covered despite similar issues. For Mars Red, sliding-scale reimbursement is available for out-of-pocket repairs. For Jupiter Red, some dealers offer 50% goodwill contributions after 3-year warranty expires. Full respray of affected panels costs €2,000-5,000 depending on extent. Specialist refinishing services offer alternatives for €1,500-3,000. Check carefully before purchase as this significantly impacts resale value.
ADAS/electronic error messages€300 - €2,000
Radar sensor dirty warnings, Pre-Safe errors, 4MATIC faults, CAN bus communication issues · more· less
The W177 relies heavily on sensors for adaptive cruise control, lane-keeping, parking assist, and Pre-Safe systems. Common issues include "Radar Sensor Dirty" messages (often during heavy rain), Pre-Safe function limited warnings, adaptive suspension errors, and wheel speed sensor failures causing 4MATIC/ABS/Brake Assist errors. Many are temporary due to water/dirt on sensors, clearing after drying. However, persistent errors can indicate sensor failure (€300-800 each), radar misalignment (€200-400 recalibration), or CAN bus communication faults (€500-2,000 diagnosis and repair). One owner had burned wiring under steering column cowling causing multiple system errors. Front grille badge and radar sensor must be kept clean. Aftermarket suspension modifications can trigger errors requiring cancellation kits. Mercedes STAR diagnostic system essential for proper diagnosis.
Turbocharger degradation€1,200 - €3,500
Turbo located between engine and bulkhead requires excellent cooling, can fail from heat stress · more· less
The M139 engine's turbocharger sits between the engine and firewall in an unusual location requiring advanced cooling - the 415hp S version uses HVAC system cooling air when needed. This positioning creates heat management challenges under sustained high-RPM operation. Watch for boost pressure error codes ("turbo 1 boost pressure too low"), sluggish throttle response, unusual whistling/grinding noises, or blue smoke from exhaust. The M139 is improved over the earlier M133, but turbo stress remains a concern on any 421hp 2.0L engine. Early oil changes every 10,000 km maximum are essential. Avoid short trips that don't allow proper warm-up/cool-down. Complete turbo replacement costs €1,200-3,500 depending on whether you need the full assembly or can repair individual components. Premium fuel (98 RON minimum) and spirited driving help prevent carbon buildup.
Water leaks into cabin€200 - €800
Blocked drainage channels, failed seals cause water in footwells and boot · more· less
Water intrusion is reported in A-Class W177 models, typically appearing in rear passenger footwells. Common causes: blocked drainage vents beneath rear tail lights (debris clogs channels), failed air pressure seals in boot/bumper area (water enters spare wheel well then flows forward), clogged AC drain pipe causing condensation backup, perished door seals, or blocked central drain under cabin filter. Symptoms include damp carpets, mold smell, and visible water pooling after rain. Check all drain tubes within vehicle structure - they can clog with leaves/debris. Remove rear tail lights to inspect drainage channels. Boot carpet should be lifted to check for water around spare wheel area. Repair typically involves clearing drains, replacing seals, or fixing boot air pressure vents (known design fault). DIY drain cleaning costs little, seal replacement €200-500, extensive work €500-800.
Brake wear (track use)€1,000 - €2,500
High-performance brakes wear quickly with spirited driving, expensive OEM Brembo parts · more· less
The A45 S uses large Brembo 6-piston front calipers with 360mm discs and 4-piston rear calipers with 330mm discs. With aggressive driving, front pads can wear in 20,000-30,000 km. OEM front brake pad and disc replacement costs €700-1,200, rear €600-900. Front discs alone are €400-600 for the pair, rears €350-500. Labor adds €300-500 depending on shop. Track use accelerates wear dramatically. Aftermarket options from Brembo, EBC, or Pagid can reduce costs but may sacrifice pedal feel or increase noise. Optional AMG carbon ceramic brakes (if equipped from factory) cost €12,000+ to replace but last much longer and handle heat better. Inspect brake disc thickness and pad material remaining before purchase. Budget €1,000-2,500 annually if driven hard, €600-1,000 for normal use.
Solid powertrain, but expect gearbox and quality issues
The M139 engine is remarkably reliable for its specific output (211 hp/liter), with many examples performing well at high mileage. However, the 8DCT gearbox has notable issues with cold engagement and long-term durability under high torque. Interior build quality is disappointing for the price point, with widespread rattle complaints. Budget significantly for high-performance consumables like brakes and tires. Essential to buy with full Mercedes service history and avoid modified cars.
Pre-Purchase Inspection Checklist
General checks
Service history
Complete Mercedes-Benz dealer or authorized AMG specialist records essential. Verify 10,000 km service intervals strictly followed.
Tires
Check tread depth (245/35R19 front, 265/35R19 rear optional), age codes, uneven wear. Michelin Pilot Sport 4S OEM spec.
Cold start
Must start engine completely cold. Listen for turbo noises, timing chain rattle, or unusual sounds in first 30 seconds.
Body condition
Inspect paint carefully, especially Jupiter Red or Mars Red colors for bubbling. Check wheel arches for damage from aggressive driving.
Test drive
Minimum 30 minutes including highway speeds to fully warm gearbox and check all systems.
Specific for this vehicle
Gearbox cold start test
With engine cold, cycle through N-R-N-D-N multiple times. Any metallic crunching (not clunk) from gearbox indicates major issue. Test reverse engagement specifically.
Drive for 20+ minutes and check gearbox operation
Feel for jerky downshifts, hesitation, or clunking during gear changes. Gearbox must shift smoothly when warm. Test in all drive modes (Comfort/Sport/Race).
Check all interior for rattles
Press dashboard center screen, air vents, door cards. Test on rough roads. Rattles are very common but indicate build quality issues.
Inspect coolant level and check for leaks
Low coolant or visible leaks around engine bay (especially below turbo area) indicate water pump issues. Check ground under car for drips.
Verify ADAS systems functioning
Test adaptive cruise control, lane keeping, parking sensors. Check for stored error messages using diagnostic scanner. Clean front radar sensor and grille.
Check paint condition thoroughly (all colors)
Inspect door handles, roof, bonnet, spoiler, A-pillars for bubbling or peeling clearcoat. Jupiter Red and Mars Red particularly prone.
Inspect for water damage in cabin
Lift floor mats, check rear footwells and boot for dampness or mold smell. Remove spare wheel cover to check for standing water.
Check for modifications or tuning
Remapped engines overstress gearbox. Look for aftermarket exhaust, intake, suspension. Ask owner directly about any ECU tune.
Brake and tire condition
Measure brake disc thickness (should be 32mm front, 22mm rear minimum). Check pad material remaining. Aggressive camber wears tires quickly.
4MATIC+ drivetrain test
Test drift mode if equipped. No error messages for 4MATIC, ABS, or ESP systems. Drive should feel balanced with no vibrations.
Recalls and Technical Service Bulletins
Paint defect - Mars Red clearcoat (590 code)Settlement active (2024)
ADAS sensor calibration updatesCheck with dealer
8DCT gearbox software updatesMultiple versions released
The W177 A45 was not sold in the United States, so US NHTSA database has limited information. Contact Mercedes-Benz with your VIN to verify all European recalls and technical service bulletins have been completed. Particularly important to check for Mars Red paint settlement eligibility and latest gearbox software version.
Warranty Status
Factory warranty (4 years/unlimited km)Expired on 2019-2020 models
Extended warranty available€2,200-7,400/year
Paint warranty (3 years)Expired on all W177
Corrosion warranty (30 years with MB service)Active if fully MB serviced
Mercedes-Benz Extended Limited Warranty offers 1-3 years additional coverage beyond factory warranty. AMG models typically cost €5,000-7,400 for longest term due to high repair costs. Note: Not all AMG models eligible (GT Black Series, SLS, SLR excluded). Given the gearbox issues, extended warranty highly recommended if buying out of factory warranty. Verify paint warranty status separately, especially for Mars Red (settlement coverage available).
This report is for informational purposes only and does not constitute professional advice. Estimates may be inaccurate. Always have a qualified specialist inspect the vehicle before purchase. We accept no liability for decisions made based on this information.